1. Field of the Invention
The present invention relates to a process for preparing oxide thin films having excellent flatness and high crystallinity, and more specifically to a reactive co-evaporation process particularly for preparing thin films of oxide superconductor materials and oxide insulator or dielectric materials, which have clean and smooth surfaces, high crystallinity and excellent superconducting or dielectric properties without any heat treatment after deposition.
2. Description of Related Art
Oxide superconductors have been found to have higher critical temperatures than those of metal superconductors, and therefore considered to have good possibility of practical application. For example, Y--Ba--Cu--O type oxide superconductor has a critical temperature higher than 80 K and it is reported that Bi--Sr--Ca--Cu--O type oxide superconductor and Tl--Ba--Ca--Cu--O type oxide superconductor have critical temperatures higher than 100K.
In case of applying the oxide superconductor to superconducting electronics including superconducting devices and superconducting integrated circuits, the oxide superconductor has to be used in the form of a thin film having a thickness of a few nanometers to some hundreds micrometers. It is considered to be preferable to utilize various deposition methods, such as sputtering methods, laser ablation methods, MBE (Molecular Beam Epitaxy) methods and reactive co-evaporation methods for forming oxide superconductor thin films. In particular, it is possible to form an oxide superconductor thin film by depositing atomic layers layer by layer through utilizing a MBE method and a reactive co-evaporation method. Additionally, in-situ observation during and between depositing thin film is possible so that a high quality oxide superconductor thin film can be obtained by the MBE method and reactive co-evaporation method.
Insulator thin films are also necessary to fabricate superconducting devices and superconducting integrated circuits. Oxide dielectrics such as SrTiO.sub.3, MgO, etc. are preferably used for insulator thin films combined with the oxide superconductor. In particular, SrTiO.sub.3 has a layered crystal structure similar to that of the oxide superconductor so that it is possible to accurately control qualities and thickness of its thin films by depositing atomic layers layer by layer through utilizing a MBE method and a reactive co-evaporation method.
In order to deposit an oxide superconductor thin film and an oxide dielectric thin film on a substrate by the MBE method and the reactive co-evaporation method, constituent elements of the oxide excluding oxygen are supplied as molecular beams towards the substrate by using Knudsen's cell (abbreviated to K cell hereinafter) type molecular beam sources. In addition, an oxidizing gas such as O.sub.2 including O.sub.3, NO.sub.2 or N.sub.2 O is supplied near the substrate so that the molecular beams are oxidized so as to form the oxide thin film on the substrate.
In general, when a thin film is deposited by the MBE method and the reactive co-evaporation method, a pressure of deposition atmosphere is reduced as low as possible so as to prevent contamination in the process. Namely, vacuum level of the deposition atmosphere is increased as high as possible.
However, in case of an oxide thin film, a above distinctive process in which an oxidizing gas is supplied near the substrate during deposition of the oxide thin film is employed. It is also preferable, even in this case, to reduce the pressure in the vicinity of the substrate as low as possible so as to prevent contamination of impurities into the oxide thin film.
For this purpose, in a prior art, the pressure in the vicinity of the substrate has been adjusted to 1.times.10.sup.-5 Torr during the deposition. However, it may be sometimes difficult to cause sufficient oxidation near a surface of the substrate.
In order to prevent diffusion of constituent elements of the substrate or a lower layer into a growing thin film, it is preferable to reduce a substrate temperature during deposition of the thin film as low as possible. However, oxidation does not sufficiently progress at the low substrate temperature. In addition, enough migration of atoms deposited on the substrate does not occur at the low substrate temperature so that a surface of the thin film becomes uneven.